A city worker removes graffiti from the base of what was once the Jackson-Lee Monument, a Confederate statue removed from Wyman Park, Baltimore on 16 August 2017. Photograph: Jim Lo Scalzo/EPA

One Friday afternoon in September 1994, a statue was pulled down from in front of provincial government offices in what was then the Orange Free State in South Africa. It depicted Hendrik Verwoerd, the country’s prime minister from 1958 to 1961, administrative architect of “apartheid” and a vicious racist. No one who grasps the barbarism that his doctrines imposed laments the removal of monuments in his honour.

Is the morality of statues honouring heroes of the Confederacy in the US civil war any more complicated? The south fought to preserve a social order founded on white racial supremacy, and economically dependent on industrial-scale slavery – a vast crime. Monuments now targeted for removal were erected not in ignorance of that atrocity but in defiant celebration of it. Their message was simple: while the law now forbids slavery, the oppression of African-Americans, their exclusion from civil rights and intimidation by a culture of casual and institutional racism will endure. It is precisely because that message has a willing audience in the US that the statues are so toxic.

It is dishonest to argue, as President Trump has done, that they belong to some protected category of national heritage. Most are relatively recent: more than 30 were erected after 2000. His suggestion that expunging Confederate symbols is a precursor to a wider liberal assault on all American icons, including George Washington, is similarly bogus. There is a necessary debate to be had about how we remember complex historical figures. But this is a diversionary tactic well practised by the US “alt-right”. It attempts to redraw the debate as an academic one between those who would purge the historical record and those who would preserve it. The deviousness of that rhetorical ploy needs exposing.

There is a valid argument for preserving artefacts that celebrate brutal regimes. Criminal violence organised on a massive scale is, sadly, a regular feature of human experience and to erase its memory sanitises history. Understanding past evil is a necessary part of our collective inoculation against its repetition. The huge death camp at Auschwitz-Birkenau was not bulldozed for that reason.

The ideal places for understanding complex history are the classroom and the museum. If villainous statues cannot be brought to halls of study, the tools of comprehension must be brought to them. Plaques can provide context and commemorate victims. Monuments can be annotated and inscriptions rewritten.

But that process is more easily conducted when a cultural consensus has been reached. The terrible problem America faces is that a view of the Confederacy as wrong and of the racist beliefs underpinning slavery as evil is, alarmingly, not settled enough. It is unclear where the president stands on a moral issue that permits no equivocation. And while the numbers of neo-Nazis might be small, they channel a sense of grievance and racial animus that fuels a much larger proportion of Mr Trump’s electoral base.

The world is littered with fragments of discredited heroes. Few warriors from the past clear the moral barriers we might set for veneration today. It is right to pause before tearing down any monuments because their deceased subjects offend later generations. But here the offence is not primarily caused by artefacts but by living, breathing political movements that would rehabilitate the vile ideas of which statues have become the rallying symbol.

Paradoxically, the case for preserving Confederate bronzes would be stronger if there were not such an aggressive ideological commitment to their preservation. That is why the events of Charlottesville have precipitated the removal of more of these statues. To tear them down is not an assault on the past. It has become a necessary defence of the present and an act of resistance to forces that would terrorise the future.